A conventional Fresnel lens is like an “accordioned” lens. It comprises annular zones, each zone approximating a slice from a parabolic or spherical surface. A cross section of a Fresnel lens typically looks like a sawtooth pattern, with each sawtooth segment starting at a base height and rising to a peak height, with the base and peak heights being approximately the same for all segments. The radial width of each segment in the lens varies, however, to accommodate the fact that the slope of the slanted face of each sawtooth segment is smaller near the center of the lens and greater near the edge of the lens.
FIG. 1 illustrates the cross section and function of a typical Fresnel lens 100. The typical Fresnel lens 100 can be highly efficient, but unless the peak height of each zone corresponds to a phase delay of precisely one wavelength (plus the phase delay gradient that reforms the incident wavefront into the desired outgoing wavefront), it is not possible for the lens 100 to direct all transmitted light to a single focus 101 because the wavefronts 103 formed by the different zones do not have exactly the correct phase relationship at every point across the lens 100 to form a desired spatially coherent wavefront 102 that converges to the single focus 101.
A conventional Fresnel zone plate 200, such as depicted in FIG. 2, is a diffractive lens formed from annular phase delay zones, each of which has a uniform phase profile. Typically the relative phase delay between adjacent zones is one wavelength (for the design wavelength of the zone plate) plus the requisite phase delay change between adjacent zones. The conventional Fresnel zone plate 200 has reduced efficiency because its output is split into multiple diffractive orders. FIG. 2 illustrates the cross section and operation of a Fresnel zone plate 200 with multiple orders focusing incoming light 204 into multiple foci 201 and 202.
A conventional blazed Fresnel zone plate 300, as illustrated in FIG. 3, combines aspects of both a Fresnel zone plate and a Fresnel lens. Every annular region in a blazed Fresnel zone plate has a phase delay that varies from zero to one wavelength, in such a way that the wavefronts 302 from adjacent zones are coherently joined edge-to-edge for converging to a common focus 301.
A disadvantage of both blazed and unblazed Fresnel zone plates is that they are fixed focal length elements, designed for focusing light of only a specific wavelength. It is desirable to have a variable focal length lens capable of being adjusted to function correctly with any selected wavelength of light.
Holographic polymer-dispersed liquid crystal devices have been demonstrated in which applying an electric field across a holographically formed active layer can switch the device between a diffractive state and a non-diffractive state.